Getting Info About Nodes

ROS2 Topics

“ROS 2 breaks complex systems down into many modular nodes. Topics are a vital element of the ROS graph that act as a bus for nodes to exchange messages.” In order to communicate, ROS2 nodes publish and/or subscribe to ROS2 topics.

Picture link

Nodes can also send messages through services and actions. If you’re interested, you could look at the official documentations:

ROS2 Services

ROS2 Actions

Setup

turtlesim

In a new terminal, run:

 ros2 run turtlesim turtlesim_node

turtle keyboard teleoperation

We'll also need something to drive the turtle around with. Let's see what other nodes are in the turtlesim package. Open a new terminal and type:

 ros2 run turtlesim<now press TAB twice>

The following should appear:

 draw_square        mimic        turtlesim_node        turtle_teleop_key

These are the four nodes in the turtlesim package. Each node is an individual executable, or program, but all are organized into a single package. Let's try the executable turtle_teleop_key. Enter:

 ros2 run turtlesim turtle_teleop_key

Reading from keyboard
---------------------------
Use arrow keys to move the turtle.
Use g|b|v|c|d|e|r|t keys to rotate to absolute orientations. 'f' to cancel a rotation.
'q' to quit.

Now you can use the arrow keys(←↑↓→) of your keyboard to drive the turtle around. Remember to select the terminal window of the turtle_teleop_key first to make sure that the keys you type are recorded.

In this case, the turtlesim node and the teleop_turtle node are communicating with each other over the ROS2 topic command_velocity: The teleop_key publishes keystrokes to the ROS2 topic /turtle1/cmd_vel, and the turtlesim node subscribes to the same topic /turtle1/cmd_vel to receive the keystroke messages.

rqt_graph

Using the rqt_graph tool we visualize the node interactions.

Unless you already have rqt installed, run:

 sudo apt update
 sudo apt install ~nros-jazzy-rqt*

In a new terminal, run:

 rqt

You will see a blank window but no worries. We need to select the rqt plugin we want to use. From the Plugins menu, select Introspection and then Node Graph. Now you should be able to see a graph of the relationship between topics and nodes.

If you place your mouse over /turtle1/cmd_vel, it will highlight the nodes (blue and green here) and topics (red here). As stated before, the teleop_turtle node and the turtlesim node are communicating on the topic named /turtle1/cmd_vel.

ros2 node

ros2 node is a useful tool for finding information about nodes.

ros2 node list

ros2 node list returns a list of all currently running nodes. This will become a more and more useful tool as you run more and more nodes.

In a new terminal run:

 ros2 node list

Assuming you still have the turtlesim and teleop_turtle nodes running, you should see:

 /teleop_turtle
 /turtlesim

ros2 node info

ros2 node info returns information about a specified node.

Example:

 ros2 node info /turtlesim

(Don't forget the forward slash "/" here.)

You should see something similar to:

/turtlesim
  Subscribers:
    /parameter_events: rcl_interfaces/msg/ParameterEvent
    /turtle1/cmd_vel: geometry_msgs/msg/Twist
  Publishers:
    /parameter_events: rcl_interfaces/msg/ParameterEvent
    /rosout: rcl_interfaces/msg/Log
    /turtle1/color_sensor: turtlesim/msg/Color
    /turtle1/pose: turtlesim/msg/Pose
  Service Servers:
    /clear: std_srvs/srv/Empty
    /kill: turtlesim/srv/Kill
    /reset: std_srvs/srv/Empty
    /spawn: turtlesim/srv/Spawn
    /turtle1/set_pen: turtlesim/srv/SetPen
    /turtle1/teleport_absolute: turtlesim/srv/TeleportAbsolute
    /turtle1/teleport_relative: turtlesim/srv/TeleportRelative
    /turtlesim/describe_parameters: rcl_interfaces/srv/DescribeParameters```_atomically: rcl_interfaces/srv/SetParametersAtomically
  Service Clients:

  Action Servers:
    /turtle1/rotate_absolute: turtlesim/action/RotateAbsolute
  Action Clients:

Note that ros2 node info allows us to see publications, subscriptions as well as services and actions associated with a certain node.

Review

• ROS2 topics: named buses over which nodes exchange messages
• rqt_graph: a tool for visualizing interactions among nodes
• ros2 node: a tool to get information about a node
  • ros2 node info: output information about a node
  • ros2 node list: output a list of available nodes